In proposed bioleaching processes for the extraction of zinc from zinc concentrates, such as described in Australian Patent No. 673,929 and International Patent Application WO 94/28184, both zinc and iron are solubilized during the bioleaching process, resulting in a solution of zinc and iron from which the zinc must then be separated.
The conventional thinking in the mining industry is that the leaching of zinc from a zinc sulphide source material requires the presence of ferric iron in solution to liberate zinc ions from the sulphide mineral lattice, according to: EQU ZnS+2Fe.sup.3+ .fwdarw.Zn.sup.2+ +S.sup.0 +2Fe.sup.2+
The ferric iron which is reduced to ferrous iron in the above process, is then regenerated to the ferric form by bacterial action.
Thus, in presently proposed zinc bioleaching processes, ferric iron is produced in solution along with zinc, which as indicated above, is considered to be a necessary requirement for effectively carrying out the process. The iron being produced in solution must then be removed in a subsequent precipitation step, prior to zinc electrowinning.
The generation of dissolved iron during bioleaching of zinc from a sulphide mineral source material, poses a number of problems in any integrated bioleach circuit:
(1) biooxidation activity is wasted since iron is being leached instead of valuable zinc; PA1 (2) dissolved iron in ferrous form will inevitably oxidize to ferric and subsequently either oxidize other sulfides or precipitate as a jarosite in a hydrolytic reaction which produces acid; and PA1 (3) dissolved iron, especially the ferric form, has a high affinity for zinc-complexing solvent extraction reagents, thus jeopardizing zinc concentration and recovery.
It is accordingly an object of the present invention to provide a bioleaching process that is selective for zinc relative to iron, whereby the extraction of iron into the leach solution is minimized.